1. Field of the Invention
The present invention relates to branched compounds and their use in sensors, particularly sensors for detecting the presence of explosives.
2. Description of the Prior Art
Semiconducting organic and organometallic (macro)molecules are promising materials for sensory devices because of their easily tunable electronic and optoelectronic properties, such as luminescence. Luminescence is the phenomenon by which light is emitted following excitation of molecules. While in theory an excited state should simply decay back to the ground state and in doing so release a photon, deactivation of the excited state can also occur by interaction with other species which will quench this luminescence. This effect can be utilised to detect analytes by measuring luminescence both before and after exposure to a substance and determining whether quenching of the luminescence has occurred.
Sensors which employ this change in luminescence in order to detect the presence of target analytes have been described previously, for example in WO-A-99/57222. This document describes polymers having characteristic luminescent lifetimes and quantum yields in isolation. However, when exposed to a target analyte which interacts with the electron distribution within the polymer after excitation, the luminescence properties change, and this can be detected as quenching of the luminescence. Analytes which can be detected by this method include any substances which cause at least partial quenching of the luminescence.
Another important example of how the optical properties of organic materials can be exploited is in devices that use amplified spontaneous emission (ASE), including lasers. In such a device, the active medium is energized, or pumped, by an external source such as light. This leads to a population inversion and stimulated emission occurs, amplifying the light coming out of the material. Analytes that interact with the active medium can alter its light amplifying properties and hence change its lasing behaviour.
There is an ongoing need to provide additional sensors which are capable of sensing different target analytes, or which are capable of sensing a plurality of target analytes, or which are capable of sensing target analytes at lower levels.